*Slice: don't mutate the underlying default value

If the default value is present, we shouldn't mutate it. Appending slices
as the code was before mutates the underlying value:

package main

import (
        "fmt"
)

func main() {
        slice := []string{"a", "b", "c"}

        fmt.Println(append(slice[:1], slice[2:]...))
        fmt.Println(slice)
}
~ go run tester2.go
[a c]
[a c c]

Let's fix this by creating and returning an entirely new slice.

Closes #1169

Signed-off-by: Tycho Andersen <[email protected]>